The present invention relates in general to a dilution headbox for ejecting stock onto a forming wire for forming a web of paper and, more particularly, to the control of dilution lines inserted into the headbox for controlling the dilution of stock flowing through the headbox and thereby the basis weight of the paper web produced.
In paper making machines, a pulp slurry referred to as stock is fed under pressure into a headbox which ejects the stock onto a forming wire through a slice opening to form a web of paper. In the past, a lip defining the slice opening has been controlled along its length to control the basis weight profile of the web of paper.
In a relatively new type of headbox control arrangement, the slice lip is not controlled. Rather, the basis weight profile of the web of paper is controlled by means of a series of dilution pipes or lines which extend across the width of the headbox. Water is controllably injected into the dilution lines to locally control the stock dilution thereby forming a variable consistency profile leaving the slice opening. By adjusting the amount of dilution at a plurality of points of the headbox across the machine, for example in response to a measured basis weight profile from on-line scanners, the basis weight profile of the web is controlled.
To precisely control the basis weight profile of a paper web being produced, it is apparent that the water flow in each of the dilution lines must be accurately and reliably controlled. In existing designs, this control has been performed by using conventional servo controlled ball valves, gate valves and needle valves with one valve and actuator per dilution line.
For proper control of dilution headboxes, the number of valves across the headboxes tends to be large, 200 or more valves being required for a dilution headbox for a wide paper making machine. Unfortunately, the conventional servo controlled valves used in existing headbox designs present serious limitations in terms of cost and reliability.
Each of the conventional valves represents a complex mechanical and electrical system including numerous moving parts which present a risk of leaks, jamming, stiction and similar problems. Further, the valves must operate in a harsh environment and conduct white water, which is used in the dilution process and contains substantial amounts of fines even when cleaned, such that deposits tend to build up within the valves. Accordingly, not only are the conventional servo controlled valves expensive, but also, under these operating conditions and with such a large number of valve systems, there is a high probability that one or more of the valves will frequently malfunction.
Another problem with conventional control valves for use in dilution headbox designs is the size of the control valves themselves. In state-of-the-art dilution headboxes, it may be desirable to have dilution lines spaced approximately every 40 mm (.apprxeq.1.6 inches) center-to-center across the headbox. Accordingly, a control device having a very narrow footprint is required and it may be impossible to achieve this spacing with conventional servo control valves.
Still another problem with conventional control valves for use in dilution headbox designs is the lack of feedback to indicate that the flow of white water through the valves is changing in response to changes in the settings of the valves. For example, a valve could be plugged or clogged with fibers such that there is no or little flow change as the valve is opened. Such operation causes control actions at a clogged valve to accumulate and possibly result in variations in the basis weight profile around that point in the paper web if the clog is partially or completely cleared as the valve is operated. Conventional flow meters are too expensive and complex to use in the large quantities required by a dilution headbox control system.
It is, thus, apparent that there is a need for an improved control system and control valves for individually controlling the dilution lines in dilution headboxes. The control system and control valves must be inexpensive, reliable and able to accurately control the flow of dilution water through the dilution lines of dilution headboxes even as the density of the dilution lines increases with the advances in the art. Further, the valves preferably could be easily maintained and serviced without shutdown of the paper making machine except for total valve replacement, for example during overhaul of the machine. It would also be desirable for the valves to provide flow signals to the control system for diagnostic and maintenance purposes without requiring the expense and complexity of conventional flow meters.